Conventional pointing devices, such as computer mice, sense motion by using opto-mechanical sensing of the motion of a ball in contact with a surface over which the mouse is being moved. The ball is situated in a cavity, and is free to rotate as the mouse is moved. The ball is also in contact with a pair of orthogonally positioned rollers, which are coupled with slotted disks. Rotation of the disks is detected optically by a Light Emitting Diode (LED) and a pair of photo detectors.
One drawback to this roller ball design is the tendency for the rollers to collect dust and dirt. This results in slippage between the ball and rollers causing unreliable motion detection. For this reason, in recent years mice have moved to direct optical sensing where the motion of the mouse is sensed relative to the surface over which it is being moved. Typically, an optical mouse has an optical sensor, similar to a low resolution camera that views the surface where the mouse is being moved. The surface is illuminated by an LED. The sensor takes pictures of the surface, detecting microscopic variations in even an apparently smooth surface. Successive frames of the surface image are compared, and motion inferred from the changes in surface features between images.
Recently, there has also been a growing market for wireless mice. Users are inconvenienced by the wire that traditionally connects the mouse to a Personal Computer (PC), and the limited freedom of movement caused by the wire. The battery life of wireless optical mice typically varies between a few weeks and a couple of months. The wireless mouse battery life is acceptable for many home users, but has resulted in limited adoption in office environments. This is partially due to Information Technology (IT) managers not wanting to continuously change mouse batteries. The greatest power drain in optical mice is generally the optical subsystem that includes both the power drawn by the LED that illuminates the mouse contact surface and the current consumption of the optical sensor.
In wired or wireless mice, the optical sensors and associated components significantly increase the cost over mice that use roller balls. In summary, wireless mice have short battery life due to the current consumption of the optical sensor and subsystem and are relatively expensive due to the cost of both the optical and wireless subsystems.